In contemporary electronic ignition systems energy is stored in the primary winding of an ignition coil to develop, upon discharge of the coil, the necessary voltage across the secondary winding to provide spark to operate the engine. The energy level is a function of the amount of current flowing through the primary and the duration thereof prior to the time when the current is interrupted to discharge the coil. It is the object of all such ignition systems to develop sufficient energy in the coil, at the highest operating engine speed, to provide the necessary voltage to cause high energy sparking. High energy spark causes the engine to operate more efficiently which reduces pollutant emissions from the vehicle.
One such ignition system provides a variable dwell time (ratio of current on time to off time) as the engine RPM is varied. Thus, at higher engine RPMs the dwell time is regulated to be longer than at lower engine RPMs which ensures sufficient energy storage in the coil to generate a high energy ignition spark. At lower engine RPMs the dwell percent is reduced in order to minimize power dissipation in the ignition circuit.
As understood, a series of alternating ignition timing signals are generated across a sensor coil in timed relationship to the engine operation. For example, for an eight cylinder automobile, eight ignition timing signals are generated to complete one cycle of the engine operation. Each individual firing cycle comprises the negative half cycle portion and the positive half cycle portion of the timing signal. In the instant ignition system the ignition timing signals are symmetrically generated about an adjustable DC potential. By comparing the magnitude of the ignition signal generated to a fixed reference potential, a triggering signal can be produced when the magnitude of the ignition signal becomes greater than the reference potential to cause current to flow through the ignition coil to effect charging of the same. In response to the engine speed increasing, the level of the DC potential is increased which effectively causes the triggering signal to be generated earlier in the firing cycle to increase the dwell percent. In this fashion, at low engine RPMs the dwell time can be maintained below 10 percent of the total firing cycle while at higher engine RPMs the dwell time can be increased to approximately 75 percent of the firing cycle. Therefore, at higher engine RPMs high energy ignition sparking is insured while at lower engine RPMs minimum power dissipation is produced.
In order to symmetrically generate the timing signal about the aforesaid DC potential, the terminal of the sensor coil which is adapted to receive the DC potential must be terminated in a very low impedance. The larger in value that this termination impedance becomes, the more that the sensor coil is effectively degained. Degaining the sensor coil can cause a misspark condition at low engine rpms which is very undesirous. Moreover, if the generated ignition signal is not symmetrical about the DC potential, control of the initiation of current flow through the ignition coil cannot be provided.
The present invention overcomes the above problems in a unique manner by providing a high impedance termination to the source of DC potential while providing a low impedance termination to the sensor coil.
To provide a low impedance termination, the prior art system uses a low value resistor to terminate the sensor coil. This allows excessive current drain to occur, as the DC potential is increased to a maximum value, at high engine rpms. However the high impedance termination of the the present invention reduces current drain of the ignition system.
Moreover, because current drain is reduced by the present invention, under "load dump" conditions, which are specified by the automobile industry, power dissipation in the present ignition system can be greatly reduced which increases the reliability of the ignition system of the invention over the prior art as described hereinafter.